This invention relates generally to cooling systems for heat-generating devices and, more particularly, to a spray cooling system and a method of using the spray cooling system to cool a heat source.
With the advent of semiconductor devices having increasingly large component densities, the removal of heat generated by the devices has become an increasingly challenging technical issue. Furthermore, typical processor boards can, in some instances, include multiple CPU modules, application-specific integrated circuits (ICs), and static random access memory (SRAM), as well as a dc--dc converter. Heat sinks can be used to increase the heat-dissipating surface area of such devices. However, heat sinks, and their interfaces to the cooled devices, can provide interference in the heat flow, and can lead to uneven cooling.
Known cooling methods for semiconductors include free-flowing and forced-air convection, free-flowing and forced-liquid convection, pool boiling (i.e., boiling a liquid cooling fluid off of a submerged device), and spray cooling (i.e., boiling a liquid cooling fluid off of a device being sprayed with the liquid). Because liquids typically have a high latent heat of vaporization, these latter two methods provide for a high heat-transfer efficiency, absorbing a large quantity of heat at a constant temperature. Typically, the cooling fluid used has a relatively low boiling point (the temperature to maintain) and is inert to the heat source. For semiconductor devices, FED. CIR.-72, i.e., Fluorinert, sold by 3M Corporation, is one of a number of known suitable cooling liquids.
The use of these boiling/vaporizing methods is limited to a maximum power density, the critical heat flux (CHF). At higher densities, the vaporized cooling fluid forms a vapor barrier insulating the device from the liquid cooling fluid, thus allowing the wall temperature of the device to increase greatly. This phenomenon is referred to as pooling. When a coolant is properly sprayed, it can disperse such a vapor layer, and its CHF can be well over an order of magnitude higher than the CHF of a pool boiling system. This high CHF is reliant on having a uniform spray. Thus, spray cooling presently provides the most efficient cooling for a heat-generating device, such as a semiconductor device.
Typically, current sprayer designs employ either pressurized liquid spraying or pressurized gas atomizing. A number of factors affect the performance of spray cooling, thus affecting the heat transfer coefficient h and/or the CHF. It is commonly understood that surface roughness and wettability of the sprayed component are two of these factors, and the orientation of the surface being sprayed can be a third. In particular, it is believed that h is higher for rough surfaces when using a pressurized liquid spray, and for smooth surfaces when using gas atomizing. Surfaces with decreased wettability appear to have a marginal increase in h.
Critical to consistent, controlled cooling is the controlled application of the liquid cooling fluid in a desired distribution, flow rate, and velocity. For example, at a low mass flow rate, CHF and h increase with the mass flow rate. However, at a critical mass flow rate, the advantages of increased mass flow are diminished due to pooling and/or due to a transition to single phase heat transfer. Thus, a spray cooling system is preferably operated uniformly at a mass flow rate defined at a point before the critical mass flow rate is reached. All of these factors make critical the design of the sprayer, i.e., the design of the nozzle and its related spray devices.
Also important to the cooling system design is its operating temperature. In particular, it is desirable to configure the system to operate at a high h, which will occur with a design temperature above the boiling temperature and below a temperature that will dry out the sprayed coolant. The amount of heat to be dissipated must be less than the CHF.
For pressure-assisted spraying, consistent, controlled spraying requires one or more high pressure pumps that provide a precise pressure to pump the liquid through a nozzle, even at varying flow rates. Both the distribution and the flow rate of the sprayed liquid can change with variations in the driving pressure and/or small variations in the nozzle construction. Thus, the cooling system is a sensitive and potentially expensive device that can be a challenge to control.
For gas atomizing, consistent, controlled spraying requires a pressurized gas that is delivered to a sprayhead design in a precise manner. Because the gas must be pressurized separately from the cooling fluid, such systems are not typically closed systems. The gas must be bled out for the condenser to run efficiently. Furthermore, both the distribution and the flow rate of the cooling fluid can change with variations in the gas pressure. Thus, the cooling system is a sensitive and potentially expensive device that can be a challenge to control.
Accordingly, there has existed a need for an accurate, reliable and cost-efficient spray cooling system. The present invention satisfies these and other needs, and provides further related advantages.